1. Field of Invention
This invention relates to an apparatus and method for friction-stir welding, and more particularly, to an apparatus and method for friction-stir welding along non-linear joints on either planar or non-planar surfaces.
2. Background of the Invention
A recent development in the welding industry, known as friction-stir welding, has revolutionized welding techniques and devices worldwide. Friction-stir welding involves plunging a non-consumable tool with a profiled, probed end into a joint line between two pieces of metal. A joint line is the small gap formed between pieces of metal being held tightly together. This gap is where the friction-stir weld occurs.
Specifically, in friction-stir welding, a weld occurs when the spinning, non-consumable tool is plunged into the joint line and forced to move along the joint line while spinning. Frictional heat is generated between the rotating tool and the pieces of metal contacted by the tool. The frictional heat softens the metal sufficiently that the stirring action of the rotating tool pulls metal across the joint line from each workpiece and fuses them together forming a weld. Though the metal is softened and plastically mixed, it is not melted. Fusion takes place below the melting temperature of the metal work pieces. The fundamental friction-stir welding process is fully described in the original patent held by The Welding Institute, U.S. Pat. No. 5,813,592.
Because the temperature of the work pieces does not reach the melting point it is possible to weld alloys that are difficult to weld by melting, such as certain aluminum alloys. There are other advantages of friction-stir welding over traditional welding. There are no fumes, spatter, and no shaded eye protection necessary for the operators. Because there is no melting, no shielding gas is needed to prevent contamination of liquid metal. There is no solidification shrinkage, so distortion of the work pieces is reduced. Friction-stir welding is also energy efficient compared to welding processes that melt metal.
Friction-stir welding offers some design advantages over traditional welding. The depth of penetration can be consistently and accurately controlled by tool geometry. Relatively deep weld joints can be achieved in a single pass. Butt, T, lap, and fillet joint configurations can be welded. Friction-stir welded joints also experience fewer traditional weld defects than joints produced by conventional welding processes.
There are also significant disadvantages with current friction-stir welding equipment and processes. The large thrust and side loads on the tool in most cases make it impossible to manually guide the weld the way one would with traditional welding processes. In friction-stir welding, the pieces to be welded are securely clamped to a rigid backing plate to resist the large side loads from the tool tending to separate the joint. This limits the current process to joints that can be clamped to a flat backing plate mounted on a large machine with heavy-duty spindle bearings for holding and rotating the tool. Most such machines are limited to tool travel in a straight line over a path no longer than the length of the backing plate to which the work pieces are clamped. Friction-stir welding would derive much benefit from a variation of the current process that would allow hand-guiding and feeding of workpieces along the rotating tool.
Four Japanese patent applications offer alternatives to the work pieces being clamped to a heavy, flat backing plate. These applications describe methods for producing circumferential friction-stir welds on large cylinders, allowing the ends of two cylinders to be joined. These variations still require heavy clamping and do not permit hand guiding of the weld along a non-straight line joint. The Japanese patent applications referred to are Japanese Pat. App. Nos. 10027652, 10141008, 11333572A2, and 10052769A2.